This is a response I sent to someone who'd asked about what kind of bike to buy for commuting, and how Bikerowave could help him get it set up.
This is my response, which has some decent info on what to look for in a used bike.
Please add additional bike buying comments / advice / etc. in this thread.
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Hello,
You have many options when it comes to putting together a commuter bike, and the Bikerowave will be happy to help.
Our stock of bikes and frames is fairly limited and variable at the moment. Your best bet is probably checking out craigslist and buying a complete used bike. (Building a bike from the frame out is tremendous fun, but the parts add up quickly, so unless you really want the experience, a complete bike is the way to go.)
For commuting, you've got a few options.
An old "ten-speed" style road bike is probably the standard way to go. Make sure it's got horizontal drop-outs and a 3 piece crank, and a singlespeed conversion can be as simple as removing the cogset from the rear wheel, removing one of the chainrings, and adding a new chain or shortening the old one.
Of course, since it is so easy, and the singlespeed / fixed gear craze shows no sign of abating, you may find it tricky to find a complete older road bike for anything like a reasonable price, since everybody wants to build a conversion. (You will see plenty of frames going cheaply though, because people get them stripped down, then realized how much it would cost to build them up into something nice, and gave up.)
If you're OK with gears, you may be able to find a newer roadbike with modern components for a reasonable sum.
If you really want a singlespeed, and you're on a budget, you may want to keep an eye out for an older, rigid frame (no suspension) mountain bike, as these are all over craigslist for less than $100. If you can find one with horizontal dropouts, a 3 piece crank and a threaded freewheel rear wheel (instead of a freehub), you can make the conversion easily, and if you add some road tires, you'll have a bulletproof commuter with plenty of character. (You can always use a chain tensioner if you can't find horizontal dropouts, but they're pretty suboptimal aesthetically.) One of these is probably going to be my next project, as I think they look really cool. (See, e.g.
http://velospace.org/files/DSCN9678.JPG ) They're not very fast though, so if that's a concern, a roadbike is preferable.
If, after reading this, you're thinking it might be good to look into something new, check some of the local bike shops (Palms Cycles, Wheel World, Performance Bike) as they sometimes have decent deals. bikesdirect.com has some adequate singlespeeds / fixies at good prices as well.
It all depends on how much time / effort / money you want to put into building the bike.
Some brands I'd try to avoid are (this is strictly a personal statement, there are plenty of people who've put plenty of miles on them, but I'd steer clear of them):
Huffy (except the Serotta Huffys, which are very desirable)
Murray
Magna
Roadmaster
Free Spirit
Pacific
Motiv
Next
GMC Denali
Iron Horse
Flying Pigeon
French bikes (the ones made in France, the ones made in Asia are usually OK)
Also try to avoid cottered cranks, 1 piece cranks, and anything that looks like an obvious basket case. Bikes are VERY EASY to get emotionally invested in, so do your homework beforehand.
If you need more info about basic bike mechanics, check out
www.sheldonbrown.com, as that's a great resource with most of the info you'll need.
Whatever you choose, Bikerowave will be happy to help you build it up or tune it up.
Good luck!